The present invention generally relates to apparatus for detecting X and Y coordinates of input points, and more particularly to apparatus for detecting X and Y coordinates of input points of handwritten characters and figures.
In the prior art, the input apparatus for handwritten characters and figures called tablets or digitizers are known. Such an apparatus includes a plurality of electrodes parallel to X and Y axes and these X and Y electrodes are coupled capacitively, magnetically or physically at an input point designated by an electric pen or a stylus, thereby the input point being detected electrically or magnetically.
In an apparatus disclosed in U.S. Pat. No. 2,907,824 to Robert Lee Peek, Jr. entitled "Electrographic Transmitter", electrically conductive wires, intersecting each other at right angles, are supported under tension, the lower wires being of a magnetic material and the upper wires of a non-magnetic material. When figures are drawn with a stylus provided with a permanent magnet, the lower magnetic wire is brought into contact with the upper wire corresponding to the locus of the stylus by the magnetic attraction, thereby generating a voltage proportional to the position of the stylus or the input point. By detecting the voltage, one learns the position of the input point. However, the prior art apparatus was seriously defective in that the stylus had to be provided with a permanent magnet at its end and an ordinary writing tool could not be used. Further, since the lower wire had to be of a magnetic material, ordinary lead wire could not be used, and the material and tension thereof needed to be selective quite severely in order to respond to the attraction of the stylus magnet. Thus, the structure became quite complex and the fine adjustment had to be made, making the whole apparatus impractical.
Of the other examples of the conventional type apparatus, one disclosed in U.S. Pat. No. 3,304,612 to Ronald R. Proctro et al has upper and lower groups of input electrodes whereby the upper group includes conductors which extend within a plurality of grooves formed in a flexible, resilient, non-conductive member and a flexible conductive sheet therebeneath mates with the non-conductive member. The lower group is of a similar structure, and the two groups of electrodes are laminated in such a way that respective conductors in the grooves of the upper electrode group intersect the respective conductors in the grooves of the lower electrode group. There are resistors connected between adjacent conductors to divide the voltage of the power source. The X and Y coordinates of a point at which the conductor and the conductive sheet in the upper and lower electrode groups are brought into contact with each other under the application of pressure can be determined by measuring the divided voltage.
In the latter conventional apparatus, it is possible to use ordinary writing tools for inputting data. However, the input surface responds to any kind of pressures other than that by the tools, for instance, the pressure of the operator's hand caused as he accidentally leans on the surface. Thus it is seriously defective in that the apparatus cannot be used with the input surface positioned horizontally. Moreover, since the conductors are applied in the grooves of a flexible insulator and the fine pitches between the grooves are difficult to determine, it is extremely difficult to achieve a high resolution. As it is extremely difficult to form such grooves with a constant pitch and over a long distance, it is practically difficult to manufacture apparatus having a large area for the input surface. In addition, the fact that both the upper and lower groups of electrodes require conductive sheets in addition to the conductors complicates the structure of the apparatus.